Ultrathin poly(ethylene glycol) monolayers formed by chemical vapor deposition on silicon substrates

Poly(ethylene glycol) (PEG) monolayer was formed through the chemisorption of a vapor phase organosilane. Photochemically cleaned silicon substrate covered with native oxide (SiO2/Si) was exposed to a vapor of an organosilane, that is, 2-(methoxy[polyethyleneoxy]propyl) trimethoxysilane (MPEOPS) for 1-7 h at 150 degrees C. The resulting sample surfaces became relatively hydrophobic showing a static water-contact angle of ca. 67 +/- 2 degrees and their thickness was ellipsometrically measured to be ca. 0.8 +/- 0.1 nm. Atomic force microscopy confirmed that the sample surface was extremely smooth and homogeneous. Isoelectric point of the MPEOPS-monolayer-covered SiO2/Si substrate was measured to be at around pH 4.9, which was close to that of the methoxytri(ethylene glycol)-terminated alkanethiol SAM on gold (pH 4.3). Furthermore, we demonstrated micropatterning of the MPEOPS monolayer based on a direct photolithography using 172 nm VUV light through a photomask. A well-defined microstructure composed of 5 microm x 25 microm rectangular features was successfully fabricated on a MPEOPS monolayer surface. Subsequently treated it by spatially defined CVD treatment using aminosilane molecule, binary microstructures composed of PEG and amino-terminated surfaces were successfully fabricated.